1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a facsimile machine, and a printer including a fixing unit for heat-fixing an unfixed image (toner image) on a transfer sheet acting as a recording medium, and a fixing control method of the fixing unit.
2. Description of the Related Art
There are widely popular image forming apparatuses in which a toner image is transferred and carried on a recording medium such as a transfer sheet, and the toner image is heat-fixed onto the recording medium by dissolving the toner included in the toner image with a fixing unit (fixing device). In such a fixing unit using the heat-fixing method, there is a pair of rotating bodies, i.e., a fixing roller (heating roller) having a built in heating source such as a heater and a pressurizing roller, which face each other with their peripheral surfaces in contact. These rotating bodies are rotated and the recording medium is passed through the rotating bodies, so that heat and pressure are applied to fix the toner.
In a fixing unit using such a heat-fixing method, pressure and heat are applied to the pair of rotating bodies while they are maintained in contact, and therefore the rotating bodies may be damaged due to the heat and pressure. Accordingly, the peripheral surfaces of the rotating bodies may be damaged or deformed. Furthermore, when the rotation of the rotating bodies is stopped for a long time, in addition to the above-described damages, an elastic layer such as rubber provided on the pressurizing roller may become deformed.
FIGS. 15A and 15B schematically illustrate a deformed state of the rotating body of such a fixing unit 100, which shows side views in the axial direction of a fixing roller 110 and a pressurizing roller 120.
When the fixing unit 100 stops operating, and is abandoned for a long time, the rollers 110 and 120 also stop rotating, and are left for a long time in a state where the peripheral surfaces of the rollers 110 and 120 are in contact while being pressurized and heated (see FIG. 15A). Accordingly, an elastic layer 121 on the peripheral surface of the pressurizing roller 120 is locally pressurized. Therefore, a part of the elastic layer 121 is deformed, centering around a contact portion (nip portion) 122 where the elastic layer 121 contacts (pressure-contacts) the high-temperature fixing roller 110, and this deformed state is maintained for a while. Consequently, permanent compression deformation may occur on the elastic layer 121 at the contact portion 122.
Thus, in the conventional fixing unit 100, when the rollers start rotating once again (see FIG. 15B), even when the contact portion 122 is released from pressure, the deformed shape at this portion does not restore its original shape. In this manner, when the rollers are stopped for a long time, the pressurizing roller 120 tends to become deformed. Furthermore, in this fixing unit 100, depending on the configuration of the fixing roller 110, deformation such as plastic deformation may occur on the fixing roller 110 itself and on a resin layer formed on the surface of the fixing roller 110, due to heat and pressure. As described above, in the conventional technology, each of the rotating bodies of the fixing unit may become deformed and damaged, thereby shortening the operating life of the fixing unit and adversely affecting the heat-fixed image.
To address such problems, there is conventionally known an image forming apparatus having the following configuration. That is, when the fixing roller and the pressurizing roller are not rotating and are being preheated, at least one of the rollers is rotated at predetermined intervals for the purpose of preventing the same portions of the rollers from being in pressure-contact with one another for a long time. In this manner, the above described deformation is mitigated while the rollers are being preheated (see patent document 1).
However, when the rollers are not being preheated, i.e., when this conventional image forming apparatus is operating under regular circumstances or during an energy saving mode, the rollers are maintained in contact with each other. Thus, after being in contact for a long time, the surfaces of the rollers may be deformed or damaged. Furthermore, there may be unevenness in the contacting position while the rollers are not rotating (the rollers may always contact each other at the same particular position). Thus, the damage caused by heat and pressure will not be evenly distributed across the rollers, but will be concentrated in the particular position. As a result, the rotating bodies may be partially damaged. Accordingly, this conventional image forming apparatus is not sufficiently effective in increasing the operating life of the fixing unit, and damages may occur within a short period of time.
There is another conventionally known information forming apparatus having the following configuration. Specifically, when the fixing unit is not operating, a pressure releasing member is inserted between the pair of rotating bodies to separate the rotating bodies from one another. Thus, the rotating bodies are prevented from being in contact for a long period of time so as to prevent the surfaces of the rotating bodies in contact with each other from becoming deformed (see patent document 2).
However, in this conventional image forming apparatus, it is necessary to additionally provide a relatively large member to be inserted between the rotating bodies, a mechanism for driving this member, and a mechanism for separating the rotating bodies. This increases the manufacturing cost as well as the size of the fixing unit and the image forming apparatus.    Patent Document 1: Japanese Laid-Open Patent Application No. 2004-333878    Patent Document 2: Japanese Laid-Open Patent Application No. 2004-264706